Dynamics and environmental regulation of the maximum leaf water use efficiency of Vitex negundo var. heterophylla during growing season

  Objective  Water use efficiency (WUE) is an important indicator of carbon and water coupling in ecosystems, and its relationship with environmental factors has attracted much attention. However, the adaptation mechanism of plant potential water use capacity to environmental change has been rarely reported. This study was to explore the seasonal dynamics of potential leaf water use efficiency and its environmental controls in Vitex negundo var. heterophylla, the dominant shrub species of Pinus tabuliformis in Beijing.

  Method  We measured the photosynthesis and transpiration of photosynthesis-light response curves and calculated corresponding instantaneous water use efficiency (WUE_i) from June to October, 2021 using a portable photosynthesis analyzer. In addition, the micrometeorological in situ observation method was used to determine the environmental factors such as air temperature (T_a), soil moisture content (SWC), air vapor pressure deficit (VPD_a) and so on. Fitting model of WUE_i against light was used to get the maximum water use efficiency (WUE_max). On this basis, the seasonal dynamics of WUE_max and the relationship between WUE_max and environmental factors were analyzed.

  Result  Consequently, the WUE_max descended first and then stayed steady during the growing season, ranging from 2.84 to9.17 μmol/mmol, with a mean of 4.79 μmol/mmol. Seasonal variations in WUE_max were mainly affected by SWC and T_a. Among them, SWC was the key environmental factor regulating WUE_max. WUE_max was negatively correlated with SWC, and SWC was mainly affected WUE_max through leaf stomatal conductance (g_c). WUE_max was positively correlated with T_a, and T_a mainly affected WUE_max by regulating photosynthetic carboxylase activity.

  Conclusion  In the humid environment, Vitex negundo var. heterophylla mainly adapts to the environment through the water resource acquisition strategy. High SWC and T_a are inhibitory factors for WUE_max in leaves of Vitex negundo var. heterophylla.